attr a1
meth init a1 := nil end
meth size($) local C = {NewCell nil} in
        C := 0
        for E in @a1 do
            C := @C+1
        end
        @C
    end
end
meth isEmpty($) if @a1 == nil then true else false end end
meth top($) case @a1 of H|T then H else raise badExpression(@a1) end end end
meth push(X) a1 := X|@a1 end
meth pop($) case @a1 of H|T then a1:=T H else raise badExpression(@a1) end end end